1. Field of the Invention
The present invention relates to membranes having internal anionic charges and processes to prepare such membranes. The anionically charged membranes are produced from membrane casting processes incorporating anionic components.
2. Background of the Technology
Anionically charge-modified membranes are useful in the removal of a variety of materials from solutions and also in certain biotechnological applications. For example, negatively charged membranes are useful in the removal of endotoxins from solutions. Endotoxins are toxic substances often derived from bacterial lysates. In addition, such membranes have found utility in the removal of positively charged species from feed-streams, such as in the preparation of ultrapure water for the semiconductor industry.
Ultrafiltration and microfiltration membranes utilized in industry, particularly in the food processing industry and in environmental applications, are typically hydrophobic membranes which may be surface-modified with a hydrophilic material to reduce fouling and to confer additional desirable properties to the membrane. Membranes may be isotropic or asymmetric (anisotropic) in their pore structure. Isotropic membranes have a uniform pore structure throughout the membrane. Asymmetric membranes do not have a uniform pore structure throughout the membrane. Asymmetric porous membranes are distinguished from isotropic, homogeneous membrane structures whose flow and retention properties are independent of flow direction. Asymmetric membranes are useful in microfiltration, ultrafiltration, and reverse osmosis processes.
Several different processes and reagents have been utilized to produce charge-modified, initially hydrophilic or hydrophobic membranes, and related membranes.
U.S. Pat. No. 4,012,324 to Gregor discloses casting formulations including a matrix polymer, a polyelectrolyte, a solvent, and a chemical cross-linking agent. Membranes are formed therefrom through a process of evaporating the solvent to form a membrane of uniform porosity and macroscopic homogeneity, having fixed anionic or cationic charges and a water content of from about 15 to about 75%. Membranes with substantial equilibrium water content are known as hydrogels and are subject to loss of water unless protected prior to use and additionally have limited application.
U.S. Pat. No. 4,673,504 to Ostreicher, et al., discloses cationic charge-modified microporous membranes that are produced from hydrophilic organic polymeric microporous membranes. These microporous membranes are hydrophilic and isotropic, with uniform pore structure throughout the membrane. However, anisotropic hydrophilic membranes are not disclosed in the Ostreicher patent.
U.S. Pat. No. 4,797,187 to Davis, et al., discloses a method to prepare ionically bonded coacervate layer membranes having improved selectivity. The Davis membranes are composite semi-permeable membranes of the type useful for reverse osmosis, gas separation, and ultrafiltration, and are post-treated to improve their selectivity.
U.S. Pat. No. 5,531,893 to Hu, et al., discloses a hydrophilic charge-modified microporous membrane having a crosslinked structure of an interpenetrating polymer network. The membrane comprises a homogeneous matrix of polyethersulfone (PES), polyfunctional glycidyl ether, and a polymeric amine such as polyethyleneimine (PEI) and like polyamines, and polyethylene glycol. A shortcoming of the '893 patent is that membranes heated for the stabilization of the network structure have a lower cationic charge density. This is stated to be due to gradual decomposition of crosslinked PEI adduct in the membrane structure.
Thus, while it can be seen that various different processes and reagents have been utilized to produce charge-modified membranes, each of the cited references has one or more undesirable features. None of the cited references produces stable, anionically charge-modified, isotropic or anisotropic, optionally non-hydrogel membranes in a simple casting process without chemical crosslinking agents. Accordingly, there remains a need for improved, stable, anionically charged membranes which possess a plurality of fixed formal negative charges that can be readily produced from polymer starting materials in a casting process without complication or expensive apparatus and which are not restricted to isotropic or hydrogel membrane types.